A typical rotary drill bit for drilling subterranean formations includes a bit body having a face region thereon carrying cutting structures for cutting into an earth formation. The bit body may be secured to a hardened steel shank having a threaded pin connection for attaching the drill bit to a drill string that includes tubular pipe segments coupled end to end between the drill bit and other drilling equipment. Equipment such as a rotary table or top drive may be used for rotating the drill string and drill bit. Alternatively, the shank may be coupled directly to the drive shaft of a down-hole motor to rotate the drill bit.
Generally, if the drill bit is a fixed-cutter, or so-called “drag” type rotary drill bit, the cutting structures on the face region of the bit body include a plurality of cutting elements formed at least in part of a superabrasive material such as polycrystalline diamond. Fixed-cutter rotary drill bits employing such polycrystalline diamond compact (PDC) cutting elements have been employed for several decades. Typically, the bit body of a rotary drill bit is formed from steel or a steel member embedded in a matrix material that includes hard particulate material, such as tungsten carbide (WC), infiltrated with a binder material such as a copper alloy.
In the case of steel body drill bits, the bit body typically is machined from stock material to the desired shape. Structural features may be defined at precise locations on the bit body by machining the bit body using a computer-controlled, multi-axis machine tool. Such structural features may include, for example, radially and longitudinally extending blades, cutting element pockets, ridges, lands, nozzle cavities, and drilling fluid courses and passages, including so-called “junk slots.” Hard-facing is usually applied to the face region of the bit body and to other critical areas of the drill bit for resisting abrasion from contact with the formation being drilled and erosion by drilling fluid during drilling operations. The cutting elements generally are secured within pockets that are machined into blades located on the face region of the bit body. The hardened steel shank may be secured to the bit body after the bit body has been formed.
Matrix-type drill bits, on the other hand, include a bit body that is at least partially formed of hard particulate material such as tungsten carbide (WC) that is infiltrated with a binder material such as a copper alloy. Matrix-type drill bits generally are formed by filling a high-temperature mold formed of graphite or a ceramic material with particulate tungsten carbide and infiltrating the particles of tungsten carbide with molten copper alloy. However, because the matrix material generally is difficult or impossible to machine, part of a machinable steel blank typically is disposed within the mold prior to infiltration of the matrix material. The infiltrant binds the steel blank to the matrix material upon hardening to form a bit body that includes both the steel blank and the matrix material. Cast resin-coated sand, graphite displacements, or in some instances tungsten carbide particles in a flexible polymeric binder, may be employed to form internal as well as external structural features of the bit body. The machinable steel blank portion of a matrix-type bit body may be secured to a hardened steel shank in the same manner described previously in relation to steel body drill bits.
FIG. 1 illustrates a conventional matrix-type drill bit 10 formed generally according to the description above. The conventional matrix-type drill bit 10 includes a bit body 12 that is coupled to a steel shank 14. A bore 16 is formed longitudinally through a portion of the drill bit 10 for communicating drilling fluid to a face 20 of the drill bit 10 during drilling operations through a plurality of passages (not shown) extending from bore 16 to the face 20, wherein typically nozzles are disposed. Cutting elements 22 and 24 (typically diamond, and most often a PDC) may be bonded to the bit face during infiltration of the bit body if thermally stable PDCs, which are commonly referred to as thermally stable products, or TSPs, are employed. Alternatively, conventional, non-thermally stable PDC cutting elements 22 and 24 having diamond tables formed on WC substrates may be bonded by the substrates to the face 20 of the bit body 12 after the bit body 12 is formed by methods such as brazing, adhesive bonding, or mechanical affixation.
The bit body 12 includes a preformed steel blank 26 and a bit body matrix 28. The bit body matrix 28 may include particles of tungsten carbide bonded together by a copper alloy. The blank 26 may have a generally cylindrical or tubular shape or a fairly complex shape that includes features for structural reinforcement of, for example, blades formed on the bit face.
During formation of the bit body 12, the blank 26 may be positioned to extend partially within a high-temperature mold for casting the bit body 12. The blank 26 is affixed to the bit body matrix 28 upon solidification of the copper alloy binder material used to infiltrate the tungsten carbide particles. An exposed upper portion of the steel blank 26 then may be machined and affixed to the shank 14 by way of a threaded connection 30 as well as by a continuous, circumferential, or “girth” weld 32 formed between the assembled shank 14 and the blank 26. The shank 14 may include tapered threads 34 forming a pin connection at an upper portion thereof for connecting the matrix-type drill bit 10 to a string of drill pipe (not shown).
After a drill bit has been manufactured, it is typically used several times to perform successive drilling operations, during which the bit body may be subjected to extreme loads and stresses due to the applied weight on bit (WOB), the applied torque used to rotate the bit, and impact forces associated with contact of the bit and cutting elements carried thereon with the subterranean formation ahead of and surrounding the well bore. These stresses may generate a defect or a plurality of defects within the drill bit and may cause existing, latent defects to grow in size. The drill bit may fail catastrophically if the characteristics and magnitudes of the defects within the drill bit reach a critical point. Such characteristics may include the nature, size, location, and orientation of individual defects, and the number of defects within the drill bit. Thus, it would be advantageous to provide a method that may be used to nondestructively inspect a drill bit after its manufacture and between successive drilling operations to identify defects within the drill bit, to characterize the nature, size, location, orientation, and number of those defects.